1. Field of the Invention
The invention relates generally to a coating composition and process for making the coating. More specifically, the invention relates to radiation-cured coatings having a macroscopic texture that provides superior abrasion resistance and unique aesthetic qualities.
2. Description of Related Art
Radiation-curable coatings are used in many applications throughout the coatings industry, such as protective coatings for various substrates, including plastic, metal, wood, ceramic, and others, and the advantages of radiation-curing compared to thermal curing are well known in the art. These coatings are typically resin-based mixtures that are usually cured using ultraviolet (UV) radiation. The resins are typically mixtures of oligomers and monomers that polymerize upon exposure to UV radiation resulting in a cured coating.
Various other components may be added to the resin mixture. A photosensitizer or photoinitiator may be added to cause cross-linkage of the polymers upon exposure to UV radiation. Flatting agents, such as silica, may be added to reduce or control the level of gloss in the cured coating; however, U.S. Pat. No. 4,358,476 discloses that excessive concentrations of flatting agents may result in undesirably high viscosities impeding proper application of the coating to a substrate, potential separation of the resin into separate phases, and a deleterious effect on the efficacy of the UV radiation. U.S. Pat. No. 5,585,415 describes the use of a pigmented composition and various photoinitiators that produce a uniform microscopic surface wrinkling that provides a low gloss surface without the use of flatting agents. Various other components, such as fillers, plasticizers, antioxidants, optical brighteners, defoamers, stabilizers, wetting agents, mildewcides and fungicides, surfactants, adhesion promoters, colorants, dyes, pigments, slip agents, fire and flame retardants, and release agents, may also be added to the resin mixture to provide additional functionality.
An important aspect of these coatings is their level of scratch or abrasion resistance. Good abrasion resistance is desirable so that the integrity and appearance of the coating is maintained. For example, a superior abrasion-resistant coating would be desirable for a flooring substrate, since flooring is typically exposed to a variety of abrasives. Improvements in the abrasion-resistance of coatings has been accomplished through various techniques. U.S. Pat. No. 4,478,876 describes the addition of colloidal silica to hydrolyzable silanes and polymers derived from a combination of acryloxy functional silanes and polyfunctional acrylate monomers. Another technique is the use of compositions containing acrylate or methacrylate functionalities on a monomer, oligomer, or resin. U.S. Pat. No. 5,104,929 describes the use of colloidal silica dispersions in certain acrylate or methacrylate ester monomers or mixtures thereof. U.S. Pat. No. 5,316,855 describes the use of a cohydrolyzed metal alkoxide sol with a trialkoxysilane-containing organic component having the trialkoxysilane.
These radiation-cured coatings generally have a substantially smooth, exposed surface such that there is no macroscopic texture or texture visible to the naked eye. This type of smooth surface provides for ease of cleaning. Some radiation-cured coatings have a microscopic texture as described in U.S. Pat. No. 5,585,415. The individual features of this texture are not visible to the naked eye, but the combined effect of the microscopic texture results in the scattering of visible light that results in a matte or low gloss appearance. This texture is provided by the coating curing process which results in microscopic wrinkles on the surface of the coating. While the microscopic dimensions of this texture provide a matte finish, these dimensions also make the coating susceptible to particle entrapment within the microscopic wrinkles. This particle entrapment results in a visibly dirty surface that is difficult to clean. Another microscopic texture found in radiation-curable coatings results from the addition of flatting agents to the uncured coating mixture. During the curing process these flatting agents, which are small inorganic or organic particles, concentrate at the coating surface to form a microscopically rough surface that scatters visible light resulting in a matte finish. The size of the particle used is typically such that it is no larger in diameter than the average thickness of the cured coating. Particles much larger than the coating thickness do not result in a matte finish and are not desired. Since most radiation-cured coatings are no more than 75-100 xcexcm thick, and since UV radiation can not typically penetrate any deeper, typical flatting agent particles for UV-cured coatings range in size from 0.1-100 xcexcm. Flatting agents are well known in the art as described, for example, in F. D. C. Gallouedec et al., xe2x80x9cOptimization of Ultrafine Microporous Powders to Obtain Low-Gloss UV Curable Coatings,xe2x80x9d Radtech Report, September/October 1995, pp 18-24.
To produce such macroscopically smooth surfaces requires the application of a coating mixture that can be easily, distributed across the substrate to be coated. If the coating mixture has a high viscosity, for example, the coating will not distribute smoothly. Therefore, it is preferable to use a lower viscosity coating to produce such a macroscopically smooth coating surface.
Other coatings provide a macroscopically textured surface by methods other than radiation-curing. In chemical embossing, for example, a macroscopic texture is formed based upon the use of various chemicals added to the substrate. In mechanical embossing, the substrate itself is imprinted with the desired textural pattern. In both types of embossing, the subsequently applied coating naturally conforms to the shape of the substrate textural pattern. However, any desired change to the textural pattern requires changes in the amount and type of chemicals added to the substrate and/or the replacement of the roller used to mechanically imprint the pattern on the substrate, which can be significantly expensive and time consuming. Furthermore, neither the coating itself or its application are inherently providing the desired texture. In another form of mechanical embossing, texture may be achieved by impressing a given pattern on the cured coating itself. Similarly, however, the texture is not produced inherently by the coating itself or its application.
Based on the foregoing, there is a need for a superior abrasion-resistant, radiation-cured coating for various substrates including plastic, metal, wood, and ceramic, among others, having a macroscopic texture. In addition, there is a need for a coating having a macroscopic texture that is easily cleanable and that provides certain aesthetic properties. Further, there is a need for a method to produce such a superior abrasion-resistant, radiation-cured coating having a macroscopic texture using a high viscosity pre-cured coating mixture and/or texture-producing particles.
In one embodiment the present invention provides a coated substrate comprising a substrate, a radiation-cured coating on at least a portion of the substrate, wherein the coating comprises an inherent macroscopic texture. In another embodiment, the present invention provides a pre-cured coating mixture comprising a radiation-curable resin and an initiator, wherein the radiation-curable resin and the initiator form a pre-cured coating mixture capable of forming a macroscopic texture upon application of the mixture on a substrate. In another embodiment the present invention provides a pre-cured coating mixture comprising a radiation-curable resin, an initiator, and texture-producing particles having an effective size to provide a macroscopic texture upon application of the mixture on a substrate. In another embodiment, the present invention provides a coated substrate comprising a substrate and a radiation-cured coating on at least a portion of the substrate, wherein the coating comprises an inherent macroscopic texture. In addition, the present invention provides a process for making a coating on a substrate, comprising the steps of distributing a pre-cured coating mixture comprising a radiation-curable resin and an initiator over at least a portion of a substrate to form a pre-cured coating having a macroscopic texture, and radiation-curing the pre-cured coating to form a radiation-cured coating having the macroscopic texture.
The coating of the present invention provides a top coat or protective coating having a macroscopic texture to substrates containing plastic such as polyvinyl chloride, metal, cellulose, fiberglass, wood, and ceramic, among others. The coating of the present invention provides superior scratch or abrasion resistance and good transparency. In addition, the coating of the present invention is easily cleanable, and the macroscopic texture provides an aesthetic aspect to the coating.
Other embodiments and features of the present invention will appear from the following description in which the preferred embodiments are set forth in detail in conjunction with the accompanying drawings.